Without trippeing a braker you can use 1500 watt,,ONE per breaker (You must find outlets on different circuits)2 of them on a 30 amp rig or several on a 50
PROBLEM.. The QUICK BOX outlets used in RVs are marginal with a 1500 watt load so suggest you not set heater to HIGH.
Solution.. I have 4 special outlets
1:Washer/Dryer (This one is a 15/20 amp and well able to haul 12 amps)
3 I added,, all 15/20 amp,,See above
Each outlet has a circuit breaker (Some 15 some 20) all to itself.. NO problems
Before I did that I had problems with hot-wire smell..Others have had failures.
AVOID ANY OUTELET POWERED BY THE INVERTER... if you have one that is.